Influencing Absorption and Desorption of Ions in Human Hair

The amorphous, keratin-associated proteins (KAPs) are a major structural component of the inner hair structure. This highly cross-linked, amorphous matrix material absorbs and desorbs water depending on the humidity of the environment. Since chemical hair treatments and oxidative hair colouring processes are mainly occurring in the matrix they influence water desorption and absorption processes. The partly crystalline keratin intermediate filaments (IFs), also referred to as microfibrils, are embedded in this amorphous matrix of KAPs. This IF-reinforced keratin composite structure is the basis for the high mechanical longitudinal strength of human hair. Therefore, the mechanical and thermo stability of the hair fiber strongly depends on KAP/IF-cohesion. Oxidative hair treatments, such as bleaching and dying, modify the KAP-matrix structure through the cleavage of disulfide bonds. Cysteic acid is formed and the protein structure becomes highly charged. As a consequence fibril/matrix-interactions decrease and hair fibre weakening occurs. Chemically treated hair appears to show an enhanced absorption of unwanted metal ions like copper and a related increased fading of artificial hair colorants. By means of treatments with specific metal ions and organic acids the cohesion of the amorphous matrix can be can also be enhanced. The keratin structure regains mechanical and thermal stability by supporting the KAP-IF interaction. Especially multivalent ions are acting here as ionic matrix-cross-linkers. This crosslink process is strongly influenced by the pH value and the choice of organic acids. The absorption of unwanted metal ions into hair and the desorption of artificial hair colours can be strongly influenced by ion-induced changes of hair matrix properties.